Superpave Use Spreads in Iowa

Asphalt Article December 28, 2000
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The city of Des Moines has implemented new asphalt technology on two of its city streets over the past two years. The 1995 construction of east 29th Street was the first project in Iowa to utilize both Superpave binder and mix design. The 1996 construction of Douglas Avenue was an overlay project, completed in one day, which utilized both Superpave binder and SHRP gradation. SHRP projects in Iowa have been the result of state, county, city and/or contractor vision. It is this vision that has allowed those involved to take the next step, hoping to further improve hot-mix asphalt. Although the mixes produced in Iowa have
improved greatly under Quality Management Asphalt (QMA), the research results of Superpave are promising enough to experiment with.

29th Street project

The 1995 project was made possible by the joint efforts of Public Works director John Bellizzi and Des Moines Asphalt and Paving Co.

The
project, which began Aug. 21, 1995, and was completed Sept. 13,
1995, is located on east 29th Street. This roadway was
completely rehabilitated. The existing curbs and gutter were
removed and the intakes rebuilt. The power poles were relocated
and water stop boxes installed. The existing asphalt base was
cold milled, driveways replaced, handicap ramps built and SHRP
binder and a 2-in. SHRP wearing surface placed.

The Des
Moines Public Works Department was responsible for approximately
70% of the work leaving 30% of the work for the Des Moines
Asphalt and Paving Co--this involved cold milling and placement
of the wearing surface. The total contract amount equaled
$225,000, with $150,000 going to the public works department.

The key element of the SHRP design system is that the
mixture is tailored to unique performance requirements dictated
by traffic, the environment and structural section. The binder
for this project was rated PG58-34, which makes the mixture
adequate for pavement temperatures up to 58 deg C and down to
-34 deg C.

To achieve this specification the asphalt had
to be modified. The key to the strength of the mix used is the
stone-on-stone contact obtained, as well as the high film
thickness.

After the milling process and preparatory work
was completed, a new 18-in. curb and gutter was installed. The
SHRP binder was then placed by the Des Moines Public Works
Department. This lift, which was approximately 600 tons, was
placed to correct the crown and shape of the existing roadway.
At that point the contractor placed the wearing surface with
double pavers operating in tandem to achieve a full-width
pavement surface with a hot centerline joint. The project is
therefore designated as seamless.

The contractor utilized
some new equipment in the paving process. A Dynapac CC501 50
series roller, was used along with a Caterpillar PS-300B
pneumatic roller. At the time of the project the Cat PS-300B was
a prototype model on loan from Caterpillar through its
field-follow program. Since January 1996 the roller has been
available commercially. The pavers used were Cedarapids CR551,
with the asphalt being loaded from a Floy-Boy and standard dump
trucks.

The project involved the paving of 3,340 lin ft
and used 600 tons of asphalt binder and leveling course, and
1,400 tons of wearing course.

Douglas Avenue project

The 1996 project consisted of patching and overlaying one
mile of Douglas Avenue. The overlay work was to be completed on
consecutive Sundays with the binder course on the first Sunday
and the surface course the next Sunday. AC-10 (similar to a
PG58-28 binder) was specified, and the state assisted by
conducting 109 gyrations on the gyratory compactor over the
course of the project. Superpave gradation was used for both the
surface and binder course.

Prior to Des Moines Asphalt
and Paving moving in, Midwest Contractors did the concrete
patching, and The Underground Co. did the intake repairs. The
project was now ready for milling, which was completed by the
paving contractor. The asphalt base course (3,000 tons at a
depth of 2 in.) and asphalt surface course (also 3,000 tons at a
depth of 2 in.) were ready to be placed. The traffic count on
this stretch of Douglas Avenue was over 18,000 vehicles per day.
In order to reduce the inconvenience to the traveling public,
the state decided to close the street completely, allowing the
contractor higher production over a shorter time. It was also
decided to use a high performance SHRP mix in the surface course
without additional cost, to accommodate the high amount of
traffic and public visibility.

Des Moines Asphalt and
Paving assembled two pavers, two asphalt plants, seven rollers
and 20 trucks to tackle the job. The two pavers, a Cedarapids
CR551, ran off of a ski, and the Roadtec 230, which used sonic
sensors, were operated in tandem with over 250 tph of
production. Paving began at 6:00 a.m. Sunday morning, was
finished at 4:00 p.m., and the street was opened for traffic
later Sunday evening after the striping had been completed by
Dennis Parking Lot Maintenance.

Although a tight schedule
was kept, the contractor received 100% of the smoothness
incentive. This is another example of efficient, high-production
paving without sacrificing quality. Since the first Superpave
project in Des Moines in 1995, others have followed throughout
the state. Some of these include: nine miles of resurfacing with
PG-graded binders of U.S. 71 in Sac and Buena Vista counties, a
seven-mile project with SHRP gradation on Iowa 175 in Hardin
County, a 9.7-mile, full-depth project on Iowa 3 in Plymouth
County, and work on Highway 63 in Howard County.

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Overlay Init